The present invention relates generally to gas and steam turbines and, more particularly, to a steam turbine blade composed of two or more components made from different materials.
Steam turbine blades operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when blade natural frequencies become in resonance with running speed or other passing frequencies (upstream bucket or nozzle count, or other major per/rev features). The magnitude of vibratory stresses when a blade vibrates in resonance is proportional to the amount of damping present in the system (damping is comprised of material, aerodynamic and mechanical components, as well as the vibration stimulus level). For continuously coupled blades, the frequency of vibration is a function of the entire system of blades in a row, and not necessarily that of individual blades within the row.
Furthermore, for turbine buckets or blades, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade contributes to higher disk stresses and thus to potentially reduced reliability.
Several prior U.S. patents/applications relate to so-called “hybrid” blade designs where the weight of the airfoil is reduced by composing the airfoil as a combination of a metal and polymer filler material. Specifically, one or more pockets are formed in the airfoil portion and filled with the polymer filler material. These prior patents/applications include U.S. Pat. Nos. 6,854,959; 6,364,616; 6,139,278; 6,042,338; 5,931,641 and 5,720,597; application Ser. No. 10/900,222 filed Jul. 28, 2004 and application Ser. No. 10/913,407 filed Aug. 7, 2004; the disclosures of each of which are incorporated herein by this reference.